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Pavement Design

Student’s Name

University’s Name

Date

3

Abstract

In this research study, we are going to take a look at the factors that are taken into consideration when in the design of the pavements and how factors affect the quality of the pavement. The factors that we shall focus on here are the performance of the pavement, the traffic of the pavement, the roadbed soil, materials of construction, the environment of the place, the drainage, the shoulder design, and lastly the life cost analysis of the pavement. We are also going to look at the factors that affect the life of a pavement and the factors that affect the pavement’s performance period.

The primary objective of looking into these factors to find the out best conditions of these factors will help create a good design of pavement to ensure that a pavement constructed is long-lasting and its structural performance is the best for all the users. We shall also look at the pavement evaluation using the falling weight deflectometer (FWD) equipment and how it is suitable for use than other equipment. The research materials that are being used in this research are from AASHTO 1993. Every factor shall be studied about the guidelines of the AASHTO document.

The findings of this research are the PSI of the pavements immediately after construction is 4 0 for the rigid pavements and 4 5 for the flexible pavements. Another finding is that the falling weight defloctometer machine is appropriate in the evaluation of the pavement.

The conclusion is that the design of pavements is a vital factor it affects the durability of the pavement. The guidelines that are provided by ASSHTO should be followed and every factor that affects the design of the pavement should be considered before the actual design of the pavement.

TABLE OF CONTENTS
Introduction 4
Methods 5
Results 7
Pavement performance 8
Traffic 9
Roadbed soil 11
Materials of construction 12
Environment 14
Drainage 16
Shoulder design 19
Reliability 19
Lifecycle cost Analysis 20
Pavement evaluation 20
Discussion 22
Conclusion 24
References 25

Introduction

The design of pavement is a very important factor in the construction of the pavements. It is the determinant factor of every aspect of the pavement. The pavements that have been constructed in the different parts of this state and on different types of roads show a big difference in their performance, endurance, and efficiency. These differences are brought about by the difference in the design of those pavements. The pavements that have been designed in the right way are bound to stay for a longer time before there arises a need for the pavements to be resurfaced or reconstructed. Those pavements that are constructed without a proper design are bound to last for a short while and also the efficiency op such pavements are very low. The period in which these pavements last before they require resurfacing or reconstruction is very short and thus it is costly.

It is the desire of any leadership of a city or a town to have pavements that are going to last. The cost that is incurred in the construction of the pavements should also be low and lastly, any leadership would want to reduce the cost of the rehabilitation of the pavements or even have fewer cases of pavement reconstruction and resurfacing. All these three needs are dependent on the design of the pavement. If the pavement is design in the right wat the pavement is going to last. Some of the factors that make a pavement not to last for long are heavy traffic, the environment of the place, and drainage among others. If the pavement is to last long, all these factors have to be put into consideration during its design. To reduce the cost of construction and rehabilitation the design of the pavement should a good one to avoid wastages in construction.

Methods

To find out how best the factors that affect the pavement design are to be applied to come up with a design that is going to help in the construction of long-lasting and cost-efficient pavements, we shall look at each of the factors individually ensuring that every measurement is done in the research. In the determination of the performance of pavement, we shall look at the serviceability of the pavement. The serviceability of the pavement will be expressed in the present serviceability index(PSI) which will be obtained from the measurement of roughness and distress. The PSI will range from 0 to 5 with zero representing a poor index and five representing a good index.

When we focus on the roadbed soil we shall look at the resilient modulus of the soil in the different seasons of the year. The resilient modulus will be measured in the lab using a triaxial resilient modulus machine for soil, abound base materials, and subbase materials. In the determination of the materials to be used in the construction, of the pavement, we shall concentrate on the stress state of the ground in which the pavement is to be constructed. The strength of the granular base which is related to the stress state which will occur under the operation conditions of the pavement shall be determined. To determine the stress state of the ground we shall find the sum of the principal of the stresses. We shall also convert the CBR or the R-value to the resilient modulus.

For the environment of the place in which the pavement is to be constructed, we shall look at the temperature and the rainfall of that place. We shall determine the effect of the temperature on the properties of asphalt, the stress caused by the temperature on asphalt, the contraction and expansion and contraction of portland cement caused by temperature change and lastly the freezing and thawing of the roadbed soil due to temperatures. We shall also determine the effect of rainfall on the properties of the materials that are used in the construction of the pavement.

In the evaluation of the pavement, we are going to use the Falling weight defloctometer where we are going to find the maximum deflection. To find the maximum deflection we shall find the ratio of the deflection at a distance from the load to the deflection under the centre of the test loads. We shall fix the distance from the load depending on the type of structure and more so preferably so that the ration will be 0.5

Figure 1: Pavement performance trends

Figure 2: Relationship between serviceability structural number and traffic(AASHTO)

Results

Method of design

The method of design of pavement is dependent on some items. These factors are to be taken into consideration in the determination of the method of design. We shall look at nine factors that are to be taken into consideration in the method of design. These factors include pavement performance, traffic of the pavement, roadbed soil, materials of construction, the environment, the place the pavement is to be constructed, the drainage of the roadbed, and the pavement design’s reliability the life cycle cost, and the shoulder design. We shall discuss each of the factors of the method of design.

Pavement performance

In the pavement performance, we take into consideration the function performance, the safety, and the structural performance of the pavement. The safety of the pavement refers to the friction that the pavement provides at the pavement tire interface. Therefore, good pavement design should be able to provide a good level of safety. To achieve this, there should be increased friction between the tires and the pavement surface. To increase friction, the roughness of the pavement is important in the design of the pavement. Also, the change in the roughness of the pavement will affect the life cycle of the pavement.

The structural performance of a pavement refers to the conditions that would adversely affect the load-carrying capability of the pavement structure. These conditions include the occurrence of cracks, faulting, and raveling. These conditions affect the performance of the pavement and also the life cycle of the pavement. In the design of the pavement, the pavement’s structural performance should be taken into consideration to ensure that the impact of the conditions that affect the performance of the pavement is minimized to ensure a longer life cycle of the pavement better structural performance of the performance.

The functional performance of the pavement refers to how well the pavement serves the user. The functional performance of the pavement is the comfort of riding offered by the pavement. The serviceability performance of the pavement quantifies the riding comfort of pavement. This serviceability performance is expressed as the present serviceability index (PSI). To obtain the present serviceability index the roughness and the distress. However, the roughness is the dominant factor in the estimation of PSI of the pavement. The scale of PSI ranges from 0 to 5 with 5 being the highest index of serviceability.

The estimate of what the PSI of pavement will be immediately after construction is known as the initial serviceability index (Pi). The pavements’ Pi values as determined by the AASHO Road Test were 4 2 for flexible pavements and 4 5 for rigid pavements. On the other hand, the terminal serviceability index (Pt) refers to the lowest acceptable level of PSI before the resurfacing and reconstruction of the pavement becomes necessary for a particular class of highway pavement. An index of 2 0 and 3 0 is used in the design of major highway and an index of 2 0 for highways with a lower classification

Traffic

Several 18-kip equivalent single axle loads can represent the damaging effect of an axle of any mass. The determination of the equivalent single axle loads is a very important consideration for the design of the pavement structures. Some key considerations influence the accuracy of the traffic estimates and significantly affect the pavement’s life cycle.

The correctness of the load equivalency values used to estimate the relative damage induces by the axle loads of different mass and configurations is one of the key considerations that influence the traffic estimates. The second consideration is the accuracy of the traffic volume and the weight of the information used to represent the actual leading projection. The other consideration that influences the traffic estimates is the interaction of age and traffic as it affects PSI changes. The load equivalency factors are a function of the thickness of the pavement and the terminal serviceability use for the design.

The performance period and the corresponding design of traffics are a reflection of the real-life experience. The road’s performance period refers to how long the pavement is good for use before it requires rehabilitation. The traffic of all lanes for both directions of travels are to be distributed by the direction and the lane for design. For directional distribution, 50 percent of the traffic is assigned to the direction. For lane distribution, 100 percent of the traffic in one direction is assigned to each lane in that direction for structural design if the measured distributions are not available. For the prediction of future traffic, we use the past traffic history of the pavement.

The traffic of pavement may remain constant or change. The change in traffic can occur in two forms. The first change can follow a straight line (linear change) and the second is the exponential change. In the design of pavement, the designer should allow for the growth of the traffic from the time of the last traffic count. If zero or negative change of the traffic is expected ten a zero or a negative factor id to be used respectively. In the case where a linear or an exponential change is expected then a linear or exponential factor is to be used respectively to ensure that the pavement design allows for the change in traffic.

The load equivalency factor increases approximately as a function of any given axle load ratio to the standard 18-kip single load with the power raised to the fourth. The future equivalency single axle load calculation is often based on the truck factors and truck class. Poor estimates of the traffic can produce pavement significantly different from the expected performance and can cause a major increase in the cost of the specified project. Reliable information concerning the cumulative ESALs is important for the determination of pavement structure requirements for both new constructions and rehabilitation. Continuous monitoring of traffic on selected routes to compare the predicted and the actual traffic loading is important in the production of a reliable design.

Roadbed soil.

Resilient modulus issued in the characterization of the roadbed soil for the pavement design. The resilient modulus is a measure of the elastic properties of a soil recognizing certain non-linear characteristics. The resilient modulus can be used directly for the design of flexible pavement. The roadbed’s resilient modulus can either be measured in the lab using the AASHTO T274 test procedure on representative samples or back-calculated from non-destructive deflection measurement. In both tests, it is important to determine the elastic module of the roadbed soil in the different seasons of the year, wet and dry conditions. Besides, determination of the length of time in each season is to be determinate. The effective roadbed soil resilience modulus can be determined using the procedure.

The placement of roadbed soil is an important factor to consider regarding the performance of pavements. To the general reliability of the design, it is necessary to consider compaction requirements. However, there are some situations for which the design should require modifications. The basic criteria for compaction of the roadbed soil shall include appropriate density requirements. Inspection must be adequate to assure that the specified density is attained during the construction. Soils that are excessively expansive or resilient should receive special consideration. This can be achieved by covering those soils with enough/ sufficient depth of selected material to modify the effect of expansion or resilience. Resilient or expansive soils can or maybe improved by compaction of water contents to a percentage above the optimum. Expansive soils can be stabilized by using a suitable admixture such as lime or cement.

For areas with frost, frost is susceptible soils should be removed and replaced with selected non-susceptible materials. Where the soils are too expensive for economic removal, the soils can be covered with sufficient depth of suitable materials to modify the effects of the freezing and thawing problem. The problem with highly organic soils is related to their extremely compressible nature. The solution to this problem is the economical excavation and replace the highly organic soils with suitable select materials. The problem that comes with deeper and more extensive deposits have been solved by placing surcharged embankments for precautions

The soils’ drainage is particularly important where heavy flows of water are encountered, where there are frost conditions, or where the soils are susceptible to expansion or loss of strength. For drainage, additional layers of pavement for interception and collection of water

Materials of construction.

Materials used in constructing the pavement can be divided into two categories: flexible pavement and rigid pavements. To complete the design requirements for flexible pavements it may be necessary to convert the CBR value information to the resilient modulus. The strength of the granular or the subbase is related to the stress state which will occur under the operating conditions. The sum of principal stresses is a sum of each layer’s pavement thickness, lead, and resilient modulus.

Flexible pavements

Flexible pavements generally consist of a prepared roadbed underlying layers of subbase, base, and surface courses. In some cases, there is the stabilization of the base of the subbase to maximize the use of local materials. The prepared roadbed is the layer of the compacted roadbed to a specified density. The subbase portion of the flexible pavement is the portion of the pavement structure between the roadbed soil and the base course. It usually consists of the compacted granular materials for economic reasons the subbase is sometimes omitted if the roadbed soils are high quality. When the roadbed soils are relatively poor quality and the design procedure indicates that a substantial thickness of the pavement is required, several alternate designs should be prepared for structural sections with or without the subbase.

The subbase has important functions. These functions include preventing intrusion of fine-grained roadbed soils into the base course, minimizing the frost action’s damaging effects, preventing the accumulation of free water within or below the pavement structure, and providing a working platform for the construction of the structure. This is important where the roadbed soil cannot provide the necessary support.

The base course is the portion of the pavement structure that is immediately beneath the surface course. It is constructed on the subbase course and where the subbase is not used it is constructed directly on the roadbed soil. The major function of the base course is to provide structural support to the pavement. It consists of aggregates. The base material must be represented by a layer coefficient (a2) so that the actual thickness may be converted to the structural number

Rigid pavements.

Rigid pavements consist of a prepared roadbed underlying a layer of subbase and a pavement slab. The subbase may be stabilized or destabilized. In the case where the is a low volume of road design where the truck traffic is low, a subbase layer may not be necessary between the prepared roadbed and the pavement slab

The subbase of a rigid pavement structure consists of one or more compacted layers of granular or stabilized material placed between the subgrade and the rigid slab to, provide a uniform, stable and permanent support, to increase the modulus of subgrade reaction(k), to minimize the damaging effects of frost action, to prevent the pumping of fine-grained soils at joints, cracks, and edges of the rigid slabs and lastly to provide a working platform for construction of the equipment. The problem of the erosion of the subbase materials under the pavement slab at joints and the pavement edge can be solved by the use of lean materials or use of porous materials in the construction of the subbase.

For a rigid pavement the basic materials that are used in the construction of the pavement slab are, Portland cement concrete, reinforcing steel, load transfer devices, and joint sealing materials. Air entrained concrete is to be used whenever it is necessary to provide resistance to surface deterioration from freezing and thawing. With the specific given conditions of a project, the minimum cement factor should be determined based on laboratory tests and prior experience of strength and durability.

For the reinforcing steel that is to be used, it has to have surface deformations adequate to bond and develop the working stresses in the steel. The longitudinal joints should form cracks at the desired locations to enable adequate sealing. The joints may be keyed. A spacing of 16 is recommended for the longitudinal joint. The faces of abutting slabs are held by tie bars to maintain firm contact.

Environment

The environmental factors are divided into two categories, environmental factors concerning the performance of the pavement and the factors regarding the structural design of the pavement. The temperature will affect the creep properties of the asphalt concrete, thermal-induced stress in asphalt concrete, contraction and expansion of the Portland cement concrete, and lastly the freezing and thawing of the roadbed soil. The difference in the temperature and the moisture between the top and the top concrete slabs in jointed concrete pavements leads to the upward curling and warping of the slab ends which can lead to pumping and deterioration of the structure.

Rainfall if allowed to penetrate the structure will influence the properties of the materials used in the construction of the pavement. Freezing and thawing of roadbed soil is a major concern. Frost heaving of the soil within or beneath a pavement is a result of the accumulation of ice within the large voids of the soil. This occurs in the oils that have fine particles. These kinds of soils are referred to as frost susceptible. The soils that are classified as non-frost-susceptible are clean sands and gravels.

There are conditions of soil, temperature, and water that must be present simultaneously for ice segregation to occur in the subsurface materials. These conditions include the soil, which has to be frost susceptible, temperature, where the freezing temperatures must penetrate the soil, and lastly water. The should be the availability of a source of water from the underlying groundwater table or the water held in the voids of the soil particles. Among the most critical phases in the annual cycle of environmental changes affecting pavements on seasonal areas are thawing periods.

Thawing can occur in one of the two directions or both directions. These directions are top-down and bottom upward. The pavement structure surface temperature determines the direction in which the thawing is going to occur. For example, during the spring thaw melting is going to occur from the surface downward. The frost’s severity is affected by climatic factors such as air temperature, the amount of solar radiation received by wind, and precipitation. Precipitation mainly affects the moisture regime but is also causes changes in the thermal properties of the soil. Frost action due to freezing temperatures in the soil can cause both heavy and thaw weakening.

For compensation of the thaw weakening effects on pavement performance, there is a need to calculate the annual roadbed soil resilient modulus. The effective modulus used for the full 12 months will produce the same PSI as would be obtained by calculating the same change with the respective seasonal moduli. The seasonal effect of the subgrade and the granular materials, the temperature will also influence the characteristics of the asphalt concrete. The pavement’s performance is going to be affected in three ways, one is low temperature cracking, the second is fatigue cracking, and the last is rutting of the pavement surface.

Drainage

Drainage of water from the pavements is an important consideration in the design of roads. If there is poor drainage of water from the pavement structure, it leads to the excess water and the combined traffic, causing early pavement distress to the pavement structure. The water enters the pavement structure in many ways. These ways include the cracks and the joints, pavement infiltration, or groundwater from an interrupted aquifer.

When this water is trapped within the structure of the pavement, there are some detrimental effects. These effects are reduced strength of unbounded granular material, reduced strength of the roadbed soils, pumping of concrete pavements with subsequent faulting, cracking, general and shoulder deterioration, and pumping of fines with aggregates base under flexible pavements with resulting of loss of support. These effects of water on the structure of the pavement lead to a shorter life cycle of the pavement and reduces comfortability and it leads to the earlier reconstruction or rehabilitation.

However, some problems are rarely noticed and arise due to entrapped water. These problems include strapping of asphalt concrete, differential heaving over swelling soils, and frost heave. There are three different methods of treating water in pavements. The first method is to prevent water from entering the pavement. The second one is providing drainage to remove excess water which is important in ensuring that there are no trapped water particles within the structure of the pavement. The last method is by building the pavement string enough to resist the combined effect of load and water.

When we consider all possible sources of water, some measures have to be taken to ensure the protection of the structural pavement section. These measures include interception of the groundwater as well as sealing of the pavement surface. The interception of groundwater is given more attention than the sealing of the pavement surface to exclude infiltration from rain and snowmelt. As a result, a considerable amount of water always enters the substructure of the pavement making the need for some type of drainage important. To obtain adequate drainage of the pavement, the pavement design has to consider the three types of water drainage systems. These drainage systems include surface drainage, groundwater drainage and the last is structural drainage.

However, the systems that we have mentioned in the paragraph above are only effective for free water. Water held by the capillary forces of the soil and in fine aggregates cannot be drained. So during the pavement design, it is important to consider the effects of this bound moisture in the soil particles through the effect of the bound water on the properties of the material of the pavement. The pavement design should include a way of quickly removing the free water from getting soil particles.

In the design method where the pavements are built strong enough to resist the combined effect of load and water, the potentially destructive effect of the water trapped within the pavement structure is not put into consideration. Therefore, there is an increased emphasis on excluding water from the pavement structure and providing rapid drainage of the water from the pavement. However, the maintenance policies should recognize the benefits and necessity of maintaining the joint sealant and thus preventing the leaking of water into the subbase layer.

There are two general designs of the pavement subsurface drainage. The first design is the criterion for the time of drainage of the base or the subbase beginning with the flooded condition and continuing to an established acceptable level. The second design is an inflow outflow criterion, by which the drainage occurs at a rate that is greater than or equal to the inflow rate, therefore, leading to the avoidance of saturation of water within the structure of the pavement. We can remove free water by draining the free water vertically into the subgrade or laterally through a drainage layer into a pipe collector’s system. In general, the actual process of the removal of free water is a combination of the two methods.

The effect of the drainage of water on the performance of the pavement is treated as the effect of water on the properties of the layers of the pavement and the consequences to the structural capacity of the pavement. The design’s effect is considered by modifying the structural layer coefficient for the flexible roads and modifying the load transfer coefficient for rigid pavements. All these modifications are to be done as a function of the quality of the drainage i.e. the time required for the pavement to drain and the percent of the time the pavement structure is exposed to moisture levels approaching saturation.

In the case where the rehabilitation of the existing pavements is done, some questions need to be answered by the designer. These questions are, is the original drainage design adequate for the existing road? What changes need to be made to ensure that drainage inadequacies, which may contribute to the distress of the pavement structure, are corrected? If the original drainage system was adequate, have there been environmental and structural changes that have taken place from the time the pavement was built? The last question is whether the present or projected land use in areas adjacent to the road indicates that surface drainage flow patterns have changed or are likely to change, thus rendering the existing drainage facilities inadequate?

Shoulder Design

The shoulder of a highway can be defined as the portion of the roadways contiguous with the traveled way to allow the accommodation of the stopped vehicles, for emergency use, and for the support of the base and subbase of the courses. If the design of the shoulders of a pavement structure requirement is similar to those of the traveled way, then some design and rehabilitation process is to be considered applicable. The use of tied shoulders or a widened width of the paving in the lane adjacent to the shoulder is beneficial to the rigid pavements’ overall performance. The paved shoulder adjacent to the flexible pavements is for the provision of lateral support for the base and the surface courses.

Reliability

We can define the pavement design-performance process as the probability that a pavement section designed using the process will have a satisfactory performance for the design period’s traffic and environmental conditions. The selection of the appropriate level of the desired level reliability for the design of a particular structure depends on the projected level of the usage and the risk associated with constructing an initially thinner pavement structure. For larger reliability, there will an increasing requirement of the thickness of the pavement, the initial cost associated with it, and the decrease in the future distress-related costs.

The level of reliability varies from 50 percent to 99.9 percent. When considering the reliability of a design, it is important to consider the effects of compound reliability. The overall reliability is the product of the reliability of all stages. The overall standard deviation is another parameter that is associated with reliability. The selection of the overall standard deviation depends on the variability of various factors associated with the prediction model’s performance. The larger the variability of the various performance factors, the larger the standard deviation and the larger the pavement thickness. If the variability of the various performance factors is small, then the standard deviation will be low and the thickness of the pavement is going to below.

Life cycle cost analysis

The objective of this analysis is to achieve the maximum economy within a project. If a pavement is designed to last for five years and then overlaid afterward for only three years before the next overlay. Looking at this case the five-year life requires larger pavement thickness and consequently larger initial cost compared to the three-year life. The three-year life requires earlier resurfacing, earlier traffic control during resurfacing and more frequent time delays for and it will eventually lead to more maintenance costs than the five-year life strategy. The common method that is used for economic analysis is the present worth method which compares alternatives after discounting all the future costs to their present worth using the appropriate discount rate.

Pavement evaluation

In the evaluation of pavements, we can use the falling weight deflectometer (FWD). The falling weight deflectometer is a reliable simple and yet effective tool in the determination of the structural properties of pavements for roads and runways. It has characteristics that act as a representative of heavy traffic than are those of most other deflection equipment. These characteristics are a level of force (up to 6o0 kN) and a loading time of approximately 30 Ms. The determination of the deflection of the pavements and the deflection bowls is done by geophones in the center of the loaded area and at certain distances from the center. The deflection levels are not affected adversely by the loading configuration.

The pavement structure is represented by a multi-layer linear elastic system in which the materials are homogenous and isotropic and are characterized by Young modulus elasticity (E) and the poisons ratio (v). The pavement response to a test load is characterized by the maximum deflection and the shape of the deflected bowl. The shape of the deflected bowl is characterized by the ratio (Qr), of the deflection at the distance r from the load to the deflection under the test load center. The ratio is chosen over the curvatures’ radius because it can be measured more easily with existing equipment and provides equivalent information.

The pulse load is applied by a mass falling onto a set of spring that is mounted in a rigid circular plate resting on the pavement surface. Velocity transducers measure the deflection of the pavement, one is located at the centre of the loaded area and one or two at a fixed distance from the load. The transducers operate over a frequency range of 1 to 300 Hz. They are very rugged which an added advantage in field conditions. By the use of this FWD equipment, the surface reflections can be measured accurately and easily without the problems encountered with some other alternatives. The transducer signals are used to record the deflections displayed on a screen to quickly permit the operator to verify the equipment and instruments’ correct operation quickly.

For conversions from CBR value to the resilient modulus value, we can use the table.

TABLE 1 Conversion sum of principle stress to Mr

Sum of the principle stresses (psi)

Resilient modulus (psi)

100

740 X CBR or 1000 + 780 X R

30

440 X CBR or 1000 + 450 X R

20

340 X CBR or 1000 + 350 X R

10

250 X CBR or 1000 +250 X R

TABLE 2 Measure conversion

Multiply by

To Find

Length

inches (in.)

25.4

millimeters (mm)

feet (ft)

0.305

meters (m)

yards (yd)

0.914

meters (m)

miles (mi)

1.61

kilometers (km)

Area

square inches (in.2)

645.1

millimeters squared (mm2)

square feet (ft2)

0.093

meters squared (m2)

square yards (yd2)

0.836

meters squared (m2)

acres

0.405

hectares (ha)

square miles (mi2)

2.59

kilometers squared (km2)

Volume

fluid ounces (fl oz)

29.57

milliliters (mL)

gallons (gal)

3.785

liters (L)

cubic feet (ft3)

0.028

meters cubed (m3)

cubic yards (yd3)

0.765

meters cubed (m3)

Discussion

Focusing on the performance of the pavement, the initial serviceability index is 4 2 for the rigid pavements and 4 5 for the flexible pavements. These values are high because at this time the pavement is still new and the safety of the pavement is still high. The comfort of using the pavement is also good. The other reason for a high initial serviceability index is because immediately after construction there are few or even no cracks on the pavements. The roughness of the pavement is still intact and the distress is minimal. The PSI that is to be maintained in the major highways is 3 0 and 2 0 for the highways with a lower classification. The difference in the PSI values is brought about by the difference in the amount of traffic experienced on these highways. The higher classification highways experience higher traffic compared to the lower classification highways.

The temperature and the rainfall of a place affect the performance of a pavement. The reason is, these two conditions affect the properties of the materials used in the construction of the pavement. In the case where the temperature increases it follows that there will be an expansion in the particles of the materials that have been used in the construction of the pavement. This expansion can lead to the development of cracks on the pavement. Low temperatures on the other hand lead to the contraction of the particles of the materials that make up the pavement. If these contractions and expansions occur often it leads to the development of cracks in the pavement and in no time the pavement will need resurfacing. The rainfall will also cause the materials that the pavement is made up of to absorb water. If there is too much rainfall the water absorbed by the materials will be a lot and it will cause expansion. If the expansion goes beyond the elastic limit of the materials, then will be cracks or breakages.

The reliability of the pavement dependent on the thickness of the pavement. A thicker pavement will be more reliable than a thin pavement. The reason behind this is because a thick pavement means that there are more layers of materials used in its construction than a thin pavement where few layers of the materials have been used in its construction. The thick pavements have high durability and resistance to distress and cracks. This will guarantee higher reliability compared to a pavement that has few layers and is susceptible to distress and cracks.

Conclusion

In conclusion, we can say that the design of pavement is a very important factor that should be considered to get the best performance pavements. The guidelines that are provided by AASHTO should be followed for they give a better way of designing pavements. The factors that are considered in the design of pavements should be put into consideration in the design of pavements. The design of pavements can be different from place to place thus although research of the conditions of the place in which the pavement is to be built should be done.

References

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7.R. D. Holtz and W. D. Kovacs(1981). An Introduction to Geotechnical Engineering. Prentice-Hall, Inc., Englewood Cliffs, N.J.,

8.A. J. Bush Ill.(1980) Nondestructive Testing for Light Aircraft Pavements. Phase Il. U.S. Army Corps of Engineers, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss.,